The present invention relates to a recording medium for eletrothermal transfer printing, and more particularly to a recording medium used for an electrothermal transfer printing method wherein point electrodes for recording and current return electrodes are held in contact with a recording medium which is superposed on a receptor, a signal voltage is applied between both the electrodes to cause a current to pass through a portion of the recording medium, thereby generating heat in the portion to transfer a coloring component contained in the recording medium onto the receptor.
As such a recording medium for electrothermal transfer printing, there is known a recording medium wherein an electrically resistive layer is provided on one surface of an insulating base film, and a hot melt transfer ink layer is provided on the other surface of the base film. A polyester film is used as the base film, and a layer wherein an electrically conductive powder such as carbon black is dispersed in a binder such as a polyester resin is used as the resistive layer.
In the case of the above-mentioned conventional recording medium, however, the dispersibility of a conductive powder such as carbon black in the polyester resin is poor and accordingly the content of the powder cannot be increased (the upper limit of the content is at most about 30% by volume), so that the resistivity of the resistive layer cannot be lowered. For the reason, heretofore only a resistive layer having a high resistivity of not less than 2 k.OMEGA./cm could be obtained.
When the resistivity of the resistive layer is too high, it is required to raise a voltage to be applied, which causes a problem relating to apparatus and another problem that when printed images with multigradation are desired, it is difficult to produce density gradations.
It has been found that the content of a conductive powder such as carbon black can be increased by using a polyvinyl butyral resin as the binder, thereby solving the above-mentioned problems. However, the polyvinyl butyral resin is poor in an adhesiveness to a base film such as polyester film, which causes a new problem that the resistive layer is easily peeled when point electrodes for recording are moved in a sliding contact with the resistive layer during electrothermal transfer printing.
Further, in the electrothermal transfer printing method, there is a dangerousness of fire since an electric current is passed through the recording medium to generate heat. Therefore, a flame resistance is required for the recording medium used for such a printing method as usually required for electronic parts.
However, since the above-mentioned conventional recording medium is poor in flame resistance, it cannot satisfy, for instance, UL-94HB standard.
It may be considered that a flame retardant is incorporated into the resistive layer. However, when a flame retardant such as phosphorus-containing retardant or halogen-containing retardant is added to the polyester resin used in the conventional resistive layer, the resistivity of the resistive layer is increased, which causes the above-mentioned problem.
It is an object of the present invention to provide a recording medium for electrothermal transfer printing in which the resistive layer has a low resistivity and is firmly adhered to the base film.
Another object of the present invention is to provide a recording medium for electrothermal transfer printing which is endowed with a good flame resistance without raising the electric resistivity of the firmly adhered resistive layer.
These and other objects of the present invention will become apparent from the descriptions hereinafter.